Lasers may be used in a variety of industrial operations including inspecting, processing, and micro-machining substrates, such as electronic materials. For example, to repair a dynamic random access memory (“DRAM”), a first laser pulse is used to remove an electrically conductive link to a faulty memory cell of a DRAM device, and then a second laser pulse is used to remove a resistive link to a redundant memory cell to replace the faulty memory cell. Because faulty memory cells needing link removals may be randomly located, workpiece positioning delay times typically require that such laser repair processes be performed over a wide range of interpulse times, rather than at a single interpulse time.
Banks of links to be removed are typically arranged on the wafer in a straight row. The links are generally processed in a link run. During a link run, the laser beam is pulsed as a stage positioner passes the row of links across the location of a focused laser spot. The stage typically moves along a single axis at a time and does not stop at each link position. This production technique is referred to in the industry as on-the-fly (“OTF”) link processing and allows for greater efficiency in the rate at which links on a given wafer can be repaired, thereby improving the efficiency of the entire DRAM production process.
As laser pulse repetition frequencies (PRFs) and link run velocities increase, more demands are placed on the stage positioner. Stage acceleration and velocity are not increasing as fast as laser PRFs. Thus, it may be difficult to take the most advantage of forthcoming high PRF lasers (e.g., PRFs in the hundreds of kHz or MHz ranges).
Generally, the current true utilization of laser pulses in a link processing system is quite low. For example, a typical wafer including approximately 600,000 links may be processed in approximately 600 seconds. This represents an effective blow rate of 1 kHz. If this example wafer processing system uses a laser source with a 100 kHz PRF, only about one out of every hundred possible laser pulses reaches the surface of the wafer.
Dual-beam and multi-beam laser systems generally use complex laser optical subassemblies and are generally expensive to construct. Further, recent advances in laser design present problems with this approach. For example, certain high power, short pulse-width (e.g., on the order of picoseconds or femtoseconds) lasers are based on a master oscillator-power amplifier (MOPA) approach in which a mode-locked laser oscillator provides stable seed pulses at repetition rates in a range between approximately 10 MHz and approximately 100 MHz. These laser oscillators may be actively or passively mode-locked. An actively locked oscillator may permit some adjustment of its output pulse phase and/or frequency for timing purposes. However, in a passively mode-locked master oscillator, the output frequency may not be so easily modified. Thus, the laser processing system synchronizes its operation with the fundamental frequency provided by the passively mode-locked master oscillator.
A power amplifier (e.g., a diode-pumped optical gain medium) amplifies selected pulses from the master oscillator. As in typical diode-pumped Q-switched lasers, the energy of these amplified pulses is a function of the interpulse period. The true operating repetition rate (e.g., the frequency of pulses issued from the power amplifier) is typically a sub-multiple of the fundamental (e.g., master oscillator) repetition rate, and is typically about 10 to 1000 times lower than the master oscillator frequency.
For desired laser operation, the laser should fire at a constant repetition rate, with the beam positioning subsystem slaved to the laser's pulse timing. However, achieving such beam position timing while maintaining pulse placement accuracy may be quite difficult. For example, the timing window for the repetition rates mentioned above may be in a range between approximately 10 nanoseconds and approximately 100 nanoseconds. Servo control systems typically cannot guarantee high-accuracy (e.g., within 10 nm) pulse placement within such small, fixed timing windows.
Many industrial laser processing applications (such as link cutting in memory device redundancy circuits, micro-via drilling, component trimming, and material cutting or scribing) emit a high-energy laser pulse in coordination with a motion control system that positions the laser pulse on a workpiece. This coordination often uses precise timing, and depending on the motion profile of the working beam, this timing may be arbitrary. While the timing precision is used to maintain the accuracy of the processing system, the arbitrary timing of pulse commands can degrade aspects of laser performance, such as pulse width and peak power.
Many laser processing system designs have incorporated Q-switched lasers to obtain consistent pulse energies at a high pulse repetition rate. However, such lasers may be sensitive to the value of (and variation in) the interpulse period. Thus, pulse width, pulse energy, and pulse amplitude stability may vary with changes in the interpulse period. Such variations may be static (e.g., as a function of the interpulse period immediately preceding a pulse) and/or dynamic (e.g., as a function of the interpulse period history). This sensitivity is generally reduced or minimized by operating the laser processing system such that the laser is fired at a nominal repetition rate (typically below 200 kHz), with minor repetition rate deviations producing acceptable deviations in pulse characteristics.
Such an approach has typically been accomplished by controlling the desired beam trajectory such that the laser may be fired “on-demand” at the appropriate workpiece location (or to hit the location with a pulse based on known factors such as stage velocity, propagation delay, pulse build up time, and other delays) to maintain the desired pulse placement accuracy. The workpiece locations are sequenced such that the repetition rate is approximately constant. “Dummy” workpiece locations may be inserted in the processing commands to account for laser stability issues. The “dummy” workpiece locations keep the repetition rate approximately constant during idle periods, with the “dummy” pulses blocked from the workpiece by beam modulation devices such as mechanical shutters, acousto-optic modulators (AOMs), and electro-optic modulators (EOMs).